Hot Melt adhesives (or HM) are solid substances at room temperature which contain neither water nor solvent. They are applied in the molten state and solidify during cooling, thereby forming a joint which ensures attachment of the substrates to be assembled. Certain hot melts are formulated to impart the support on which it is coated, relative hardness without any tack. Other hot melts provide relative softness and significant tack to the support. These other hot melts are designated as “Hot Melt Pressure Sensitive Adhesives” (or HMPSAs) and are widely used for making self-adhesive labels.
Pressure sensitive adhesives (also-called self-adhesive glues or PSAs) are substances giving the support on which it is coated immediate tackiness at room temperature (often designated as tack), which allows them to be instantaneously adhered to a substrate under the effect of a slight and short pressure.
So-called permanent grade PSAs are widely used for making self-adhesive labels which are attached on items for the purpose of showing information (such as a bar code, a name, a price) and/or for decorative purposes.
The PSAs are generally applied by continuous coating methods over the whole of the surface of a printable supporting layer of large dimensions consisting of paper or of film of a polymeric material with one or more layers. The adhesive layer which covers the printable supporting layer is itself covered with a protective layer (often designated as “release liner”), for example consisting of silicone film or paper. The obtained multilayer system is generally packaged by winding it up in the form of wide reels having a width up to 2 meters and a diameter up to 1 meter, which may be stored and transported.
This multilayer system may subsequently be converted into self-adhesive labels which may be applied by the final user by means of transformation methods which include the printing of desired information and/or decorative elements on the printable face of the supporting layer, and then their cutting out into the desired shape and dimensions. The protective layer may easily be removed without modifying the adhesive layer which remains attached on the printable supporting layer. After separation of its protective layer, the label is applied at a temperature ranging from 0° to 50° C. on the item to be coated either manually, or with labelers on automated packaging lines.
The PSAs, because of their high tack at room temperature, allow the label to be rapidly adhered or attached to the item to be coated, which is suitable for obtaining significant industrial production rates.
Nevertheless, depending on the surface condition of the item, i.e. depending on its roughness level, depending on the formulation of the composition of the adhesive used and depending on the quality of the support of the label, a self-adhesive label does not behave in the same way over time. Before labeling, edge rolling problems (curling) involving poor positioning of the label on the item at the machine outlet may be observed. After labeling, problems of detachment of the edges of the label (in the form of a halo or “haloing”) may also be observed. These problems are detrimental to proper application and proper hold of the label during the lifetime of the article.